We present an adaptively biased molecular dynamics (ABMD) method for the computation of the free energysurface of a reaction coordinate using nonequilibrium dynamics. The ABMD method belongs to the general category of umbrella sampling methods with an evolving biasing potential and is inspired by the metadynamics method. The ABMD method has several useful features, including a small number of control parameters and an numerical cost with molecular dynamics time . The ABMD method naturally allows for extensions based on multiple walkers and replica exchange, where different replicas can have different temperatures and/or collective variables. This is beneficial not only in terms of the speed and accuracy of a calculation, but also in terms of the amount of useful information that may be obtained from a given simulation. The workings of the ABMD method are illustrated via a study of the folding of the Ace-GGPGGG-Nme peptide in a gaseous and solvated environment.

We have investigated the radial electron pair probability distributions (REPPDs) of the helium dimer within the Piris natural orbital functional (PNOF) theory. The analytical formulas to evaluate intracule densities, Fermi, Coulomb, and total correlation holes using our reconstruction functional PNOF-2 [J. Chem. Phys.126, 214103 (2007)] are derived. The Löwdin’s Coulomb holes from PNOF-2 and full configuration interaction calculations are analyzed showing a very similar behavior. New definitions of the Coulomb and Fermi holes based on the cumulant expansion of the two-particle reduced density matrix are presented. The holes are defined in terms of the exact one-particle reduced density matrix and the two-particle cumulant without any reference to the Hartree–Fock state. Through these definitions, we analyze separately the contribution of each component to the total REPPD at several values of the internuclear distance. A straight connection between the Coulomb hole and dispersioninteractions is observed.

The efficiency of the two-surface monte carlo (TSMC) method depends on the closeness of the actual potential and the biasing potential used to propagate the system of interest. In this work, it is shown that by combining the basin hopping method with TSMC, the efficiency of the method can be increased by several folds. TSMC with basin hopping is used to generate quantum mechanical trajectory and large number of stationary points of water clusters.

A reformulation of the fixed-node diffusionquantum Monte Carlo method (FN-DQMC) in terms of the -particle density matrix is presented, which allows us to reduce the computational effort to linear for the evaluation of the local energy. The reformulation is based on our recently introduced density matrix-based approach for a linear-scaling variational QMC method [J. Kussmann et al., Phys. Rev. B.75, 165107 (2007)]. However, within the latter approach of using the positive semi-definite -particle trial density , the nodal information of the trial function is lost. Therefore, a straightforward application to the FN-DQMC method is not possible, in which the sign of the trial function is usually traced in order to confine the random walkers to their nodal pockets. As a solution, we reformulate the FN-DQMC approach in terms of off-diagonal elements of the -particle density matrix , so that the nodal information of the trial density matrix is obtained. Besides all-electron moves, a scheme to perform single-electron moves within -PDM QMC is described in detail. The efficiency of our method is illustrated for exemplary calculations.

A new approach, named auxiliary density perturbation theory, for the calculation of second energy derivatives is presented. It is based on auxiliary density functional theory in which the Coulomb and exchange-correlation potentials are expressed by auxiliary function densities. Different to conventional coupled perturbed Kohn–Sham equations the perturbed density matrix is obtained noniteratively by solving an inhomogeneous equation system with the dimension of the auxiliary function set used to expand the auxiliary function density. A prototype implementation for the analytic calculation of molecular polarizabilities is presented. It is shown that the polarizabilities obtained with the newly developed auxiliary density perturbation approach match quantitative with the ones from standard density functional theory if augmented auxiliary function sets are used. The computational advantages of auxiliary density perturbation theory are discussed, too.

A comparison of chain-of-states based methods for finding minimum energy pathways (MEPs) is presented. In each method, a set of images along an initial pathway between two local minima is relaxed to find a MEP. We compare the nudged elastic band (NEB), doubly nudged elastic band, string, and simplified string methods, each with a set of commonly used optimizers. Our results show that the NEB and string methods are essentially equivalent and the most efficient methods for finding MEPs when coupled with a suitable optimizer. The most efficient optimizer was found to be a form of the limited-memory Broyden-Fletcher-Goldfarb-Shanno method in which the approximate inverse Hessian is constructed globally for all images along the path. The use of a climbing-image allows for finding the saddle point while representing the MEP with as few images as possible. If a highly accurate MEP is desired, it is found to be more efficient to descend from the saddle to the minima than to use a chain-of-states method with many images. Our results are based on a pairwise Morse potential to model rearrangements of a heptamer island on Pt(111), and plane-wave based density functional theory to model a rollover diffusion mechanism of a Pd tetramer on MgO(100) and dissociative adsorption and diffusion of oxygen on Au(111).

We show using two simple nonlinear quantum systems that the infinite set of quantum dynamical variables, as introduced in quantized Hamilton dynamics [O. V. Prezhdo and Y. V. Pereverzev, J. Chem. Phys.113, 6557 (2000)], behave as a thermostat with respect to the finite number of classical variables. The coherent classical component of the evolution decays by coupling to the chaotic quantum reservoir. The classical energy, understood as the part of systemenergy expressible through the average values of coordinates and momenta, is transferred to the quantum energy expressible through the higher moments of coordinates and momenta and other quantum variables. At long times, the classical variables reach equilibrium, and the classical energy fluctuates around the equilibrium value. These phenomena are illustrated with the exactly solvable Jaynes–Cummings model and a nonlinear oscillator.

We present projected gradient algorithms designed for optimizing various functionals defined on the set of -representable one-electron reduced density matrices. We show that projected gradient algorithms are efficient in minimizing the Hartree-Fock or the Müller-Buijse-Baerends functional. On the other hand, they converge very slowly when applied to the recently proposed functionals [O. Gritsenko et al., J. Chem. Phys.122, 204102 (2005)]. This is due to the fact that the functionals are not proper functionals of the density matrix.

We derive an efficient method for the insertion of structured particles in grand canonical Monte Carlo simulations of adsorption in very confining geometries. We extend this method to path integral simulations and use it to calculate the isotherm of adsorption of hydrogen isotopes in narrow carbon nanotubes (two-dimensional confinement) and slit pores (one-dimensional confinement) at the temperatures of 20 and 77 K, discussing its efficiency by comparison to the standard path integral grand canonical Monte Carlo algorithm. We use this algorithm to perform multicomponent simulations in order to calculate the hydrogen isotope selectivity for adsorption in narrow carbon nanotubes and slit pores at finite pressures. The algorithm described here can be applied to the study of adsorption of real oligomers and polymers in narrow pores and channels.

A benchmark set of 28 medium-sized organic molecules is assembled that covers the most important classes of chromophores including polyenes and other unsaturated aliphatic compounds, aromatic hydrocarbons, heterocycles, carbonyl compounds, and nucleobases. Vertical excitation energies and one-electron properties are computed for the valence excited states of these molecules using both multiconfigurational second-order perturbation theory, CASPT2, and a hierarchy of coupled cluster methods, CC2, CCSD, and CC3. The calculations are done at identical geometries and with the same basis set (TZVP). In most cases, the CC3 results are very close to the CASPT2 results, whereas there are larger deviations with CC2 and CCSD, especially in singlet excited states that are not dominated by single excitations. Statistical evaluations of the calculated vertical excitation energies for 223 states are presented and discussed in order to assess the relative merits of the applied methods. CC2 reproduces the CC3 reference data for the singlets better than CCSD. On the basis of the current computational results and an extensive survey of the literature, we propose best estimates for the energies of 104 singlet and 63 triplet excited states.

Here, we introduce a simple self-adaptive computational method to enhance the sampling in energy, configuration, and trajectory spaces. The method makes use of two strategies. It first uses a non-Boltzmann distribution method to enhance the sampling in the phase space, in particular, in the configuration space. The application of this method leads to a broad energy distribution in a large energy range and a quickly converged sampling of molecular configurations. In the second stage of simulations, the configuration space of the system is divided into a number of small regions according to preselected collective coordinates. An enhanced sampling of reactive transition paths is then performed in a self-adaptive fashion to accelerate kinetics calculations.

We present a dc sliced ion imaging study of HCCO radical photodissociation to CH and CO at . The measurements were made using a two-color reduced Doppler probe strategy. The CO rotational distribution was consistent with a Boltzmann distribution at . Using the dc slice ion imaging approach, we obtained CO images for various rotational levels of CO . The results are largely consistent with earlier work, albeit with a significant peak seen previously in the translational energy distributions absent in our state-selected imaging study.

New experimental and theoretical results are presented that address the movement of ions through argon gas. On the experimental front, improved ion mobility results are presented. These results confirm the presence of the oft-cited mobility minimum as a function of electrostatic field strength at room temperature. On the theoretical side, high-level ab initio potential energy curves are calculated for the system and, from these, transport properties are calculated and compared to experiment. A crossing between the lowest curve and the ground state curve near the minimum of each potential becomes an avoided crossing on the inclusion of spin-orbit coupling. It is shown that the more appropriate potential for the description of the motion of through Ar at the energies of interest is the diabatic potential, neglecting fine structure. By using an improved potential, agreement with the mobility measurements is obtained for low and intermediate electrostatic field strengths, although small discrepancies remain for high field strengths. The appropriate choice of diabatic or adiabatic potentials is also considered for related systems of interest: , , and .

The dissociationdynamics of molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from to and and parallel transition from to , involving two product channels Br and Br , respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of molecules on the three excited states can be changed to different degrees. The branching ratio of is calculated as a function of pulse delay time and phase difference.

A recently developed method for calculating anharmonic vibrational energy levels at nonstationary points along a reaction path that is based on second-order perturbation theory in curvilinear coordinates is combined with variational transition state theory with semiclassical multidimensional tunneling approximations to calculate thermal rate constants for the title reaction. Two different potential energy surfaces were employed for these calculations, an improved version of the author’s surface 5 and the WSLFH surface of Wu et al. [J. Chem. Phys.113, 3150 (2000)]. We present detailed comparisons of rate constants computed for the two surfaces with and without anharmonicity and with various approximations for incorporating tunneling along the reaction path. The results for this system are quite sensitive to the surface employed, the choice of coordinates (curvilinear versus rectilinear), and the inclusion of anharmonicity. A comparison with experiment provides information on the accuracy of these surfaces.

The properties of a number of states of calcium are determined from a large basis configuration interaction calculation. The main focus is on the polarizabilities of the low lying states (the , , , and states) and the dispersion interactions of those states with the calciumground state, the hydrogen atom, and the rare gases.

Electron detachment from fullerene dianions stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and . The fraction of surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.

The electronic structure of both and has been calculated at the density functional theory (DFT) level, employing the zero order regular approximation at the scalar relativistic level and including a spin-orbit coupling. The effect of the inclusion of the spin-orbit coupling is discussed, and the differences assigned to the nature of the encaged atom (W or Mo) are identified. Then, the excitation spectra of both clusters are calculated at the time-dependent DFT level, also in this case at both scalar relativistic and spin-orbit levels. The inclusion of spin-orbit coupling is mandatory for an accurate description in the low energy region. At higher energy, where the density of states is higher, the convoluted intensity can be properly described already at the scalar relativistic level. The consequences of the spin-orbit coupling on the excitation spectrum of the clusters indicate that while in the lowest excitations are essentially shifted in energy with respect to the scalar relativistic results, in , a dramatic splitting in many lines is actually predicted, revealing a quite different behavior of the two clusters.

The dissociative recombination of and has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be and , respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for at interaction energy and are determined to be , and . These values have been obtained assuming that the rearrangement channel forming is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.

We have studied the fragmentation dynamics of core-excited by means of soft-x-ray photoexcitation and partial positive and negative ion yield measurements around the Si -shell and F -shell ionization thresholds. All detectable ionic fragments are reported and we observe significant differences between the various partial ion yields near the Si threshold. The differences are similar to our previous results from showing more extended fragmentation in correspondence to transitions to Rydberg states. At variance with smaller systems, we observe negative ion production in the shape resonance region. This can be related to the possibility in a relatively large system to dissipate positive charge over several channels.